The present invention provides for adsorption processes using adsorption zones which contain adsorbent beads of different geometrical shapes and sizes and monolithic adsorbents. More particularly the present invention provides for pressure and/or vacuum swing adsorption processes using these adsorption zones.
Cyclic adsorption processes are frequently used to separate the components of a gas mixture. Typically, cyclic adsorption processes are conducted in one or more adsorbent vessels that are packed with a particulate adsorbent material which adsorbs at least one gaseous component of the gas mixture more strongly than it adsorbs at least one other component of the mixture. The adsorption process comprises repeatedly performing a series of steps, the specific steps of the sequence depending upon the particular cyclic adsorption process being carried out.
In any cyclic adsorption process, the adsorbent bed has a finite capacity to adsorb a given gaseous component and, therefore, the adsorbent requires periodic regeneration to restore its adsorption capacity. The procedure followed for regenerating the adsorbent varies according to the process. In VSA processes, the adsorbent is at least partially regenerated by creating vacuum in the adsorption vessel, thereby causing adsorbed component to be desorbed from the adsorbent, whereas in PSA processes, the adsorbent is regenerated at atmospheric pressure. In both VSA and PSA processes, the adsorption step is carried out at a pressure higher than the desorption or regeneration pressure.
A typical VSA process generally comprises of a series of four basic steps that includes (i) pressurization of the bed to the required pressure, (ii) production of the product gas at required purity, (iii) evacuation of the bed to a certain minimum pressure, and (iv) purging the bed with product gas under vacuum conditions. In addition a pressure equalization or bed balance step may also be present. This step basically minimizes vent losses and helps in improving process efficiency. The PSA process is similar but differs in that the bed is depressurized to atmospheric pressure and then purged with product gas at atmospheric pressure.
As mentioned above, the regeneration process includes a purge step during which a gas stream that is depleted in the component to be desorbed is passed countercurrently through the bed of adsorbent, thereby reducing the partial pressure of adsorbed component in the adsorption vessel which causes additional adsorbed component to be desorbed from the adsorbent. The nonadsorbed gas product may be used to purge the adsorbent beds since this gas is usually quite depleted in the adsorbed component of the feed gas mixture. It often requires a considerable quantity of purge gas to adequately regenerate the adsorbent. For example, it is not unusual to use half of the nonadsorbed product gas produced during the previous production step to restore the adsorbent to the desired extent. The purge gas requirement in both VSA and PSA processes are optimization parameters and depend on the specific design of the plant and within the purview of one having ordinary skill in the art of gas separation.
Many process improvements have been made to this simple cycle design in order to reduce power consumption, improve product recovery and purity, and increase product flow rate. These have included multi-bed processes, single-column rapid pressure swing adsorption and, more recently, piston-driven rapid pressure swing adsorption and radial flow rapid pressure swing adsorption. The trend toward shorter cycle times is driven by the desire to design more compact processes with lower capital costs and lower power requirements. The objective has been to develop an adsorbent configuration that demonstrates an ability to produce the required purity of product, with minimum power consumption and lower capital costs.
Monolithic adsorbents offer a number of advantages, particularly low pressure drop which translates into higher power savings. Other advantages include excellent attrition resistance, good mechanical properties, compactness, and no fluidization constraints. Monolith beds have faster mass transfer rates and perform close to equilibrium conditions. However, due to the low pressure drop across monolithic beds, the flow distribution of the feed inside the bed may not be as good as a granular bed. Historically, an empty chamber at the bottom of an adsorbent vessel has been employed to achieve a good flow distribution of feed through the adsorbent layer. However, this may not work with monolith beds since the parallel channels in the monolith are not connected.
Additionally, permeability in the axial direction (i.e. direction of gas flow) in some monolithic beds is quite high and any abnormality in the channels may produce a maldistribution of flow. This leads to process underperformance and may prevent reaching the desired process purity values. This inefficiency may make the use of monolithic beds, which are more expensive than conventional packed beds, less desirable.
The present inventors have discovered bed configurations which will maintain good flow distribution and maintain process efficiency at the same time.
An adsorption process employing composite beds with adsorbent materials of different geometrical shapes, particle sizes and adsorption properties is disclosed. The process which can be vacuum swing adsorption(VSA), pressure swing adsorption process (PSA) or pressure-vacuum swing adsorption(PVSA) is used to separate oxygen from air or other oxygen-containing gases. The composite bed will typically contain a layer of monolith adsorbent material and a layer of adsorbent beads.
Optimized bed performance is achieved by the configuration and by carefully designing the amount of each type of different size, shape and property of adsorbent material in each layer of the bed. The keys are to achieve reduced pressure drop and maintaining higher product throughput without fluidization and distribution problems.